Microphones are used in a variety of different devices and applications. For example, microphones are used in headsets, cell phones, music and sound recording equipment, sound measurement equipment and other devices and applications. In one particular application, headsets with microphones are often employed for a variety of purposes, such as to provide voice communications in a voice-directed or voice-assisted work environment. Such environments use speech recognition technology to facilitate work, allowing workers to keep their hands and eyes free to perform tasks while maintaining communication with a voice-directed portable computer device or larger system. A headset for such applications typically includes a microphone positioned to pick up the voice of the wearer, and one or more speakers positioned near the wearer's ears so that the wearer may hear audio associated with the headset usage. Headsets may be coupled to a mobile or portable communication device that provides a link with other mobile devices or a centralized system, allowing the user to maintain communications while they move about freely.
Work environments in voice-directed or voice-assisted systems are often subject to high ambient noise levels, such as those encountered in factories, warehouses or other worksites. High ambient noise levels may be picked up by the headset microphone, masking and distorting the speech of the headset wearer so that it becomes difficult for other listeners to understand or for speech recognition systems to process the audio signals from the microphone. To maintain speech intelligibility in the presence of high ambient noise levels, it is therefore desirable to increase the ratio of speech energy to ambient noise energy—or the signal to noise ratio (SNR)—of the audio transmitted from the headset by reducing the sensitivity of the microphone to ambient noise levels while maintaining or increasing its sensitivity to the acoustic energy created by the headset wearer's voice.
Microphones designed to suppress ambient noise in favor of user speech are commonly known as noise cancellation microphones. One type of noise cancellation microphone is a dipole microphone, which is also sometimes referred to as a bi-directional, or figure 8 microphone. Unlike an omni-directional microphone, which is strictly sensitive to the absolute air pressure at the microphone, a dipole microphone generates output signals in response to air pressure gradients across the microphone.
High quality dipole microphones may be constructed using a single element, such as a ribbon or diaphragm. To make the microphone sensitive to pressure gradients, both sides of the diaphragm are exposed to the ambient environment, so that the diaphragm moves in response to the difference in pressure between its front and back. Acoustic waves arriving from the front or back of the diaphragm will thus be picked up with equal sensitivity, with acoustic waves arriving from the back producing output signals with an opposite phase as those arriving from the front. In contrast, acoustic waves arriving from the side produce equal pressure on both the front and back of the diaphragm, so that the diaphragm does not move, and thus the microphone does not produce an output signal. For this reason, a well designed single-diaphragm dipole microphone may have a deep response null to acoustic waves arriving at an angle of 90° degrees to the forward or reverse pickup axes.
Although single element dipole microphones may offer excellent performance, they are expensive, which can drive up the cost of devices, such as headsets, employing them as a noise cancelling microphone. A less costly way of constructing a dipole microphone is to space two lower cost omni-directional acoustic sensors a distance apart, and electrically connect the sensors so that their output signals are added together out of phase. Acoustic waves causing a pressure gradient across the dipole pair—such as acoustic waves arriving lengthwise with respect to the dipole pair—will result in each acoustic sensor generating a different output signal, so that the resulting differential output of the dipole pair will be non-zero. Acoustic waves that produce the same absolute pressure at each acoustic sensor—such as acoustic waves arriving from the side, or low frequency far field acoustic waves—will cause each omni-directional acoustic sensor to produce the same output signal so that the resulting differential sum is zero. Thus, similarly to a single element dipole microphone, a dipole microphone consisting of a pair of omni-directional acoustic sensors is sensitive to the pressure gradient across the microphone rather than the absolute sound pressure level at the microphone.
The pressure gradient sensitivity of a dipole microphone makes it particularly well suited for use as a noise cancelling microphone on a headset. Because a headset microphone is typically in close proximity to the wearer's mouth, the microphone is in what is commonly referred to as a near field condition with respect to the wearer's voice. Near field conditions typically result in acoustic waves that are generally spherical in shape with a small radius of curvature when in close proximity to the source of the acoustic energy. Because a spherical acoustic wave's intensity has an inverse relationship to the logarithm of the distance from the source, the sound pressure at each acoustic sensor of a multi-element dipole microphone in this near field condition may be substantially different, creating a large pressure gradient across the microphone. As acoustic waves propagate a greater distance from their source, the sound pressure in the wave does not decrease as rapidly over a given distance, such as the distance between the acoustic sensors of a multi-element dipole microphone. Therefore, a much smaller pressure gradient is created across the microphone by acoustic waves originating from more distance sources, so that the microphone is generally less sensitive to these distant sources.
The pressure gradients generated across the microphone are also affected by the phase difference between the acoustic waves arriving at the two acoustic sensors. Because the acoustic sensors are separated by a short distance, the sound pressures at each sensor will have a phase difference that depends in part on the wavelength of the incident acoustic wave. Acoustic waves having shorter wavelengths will thus generally cause the microphone to experience a higher degree of phase difference between the acoustic sensors than lower frequency waves, since the distance separating the sensors will be a larger fraction of the higher frequency wavelength. Because—for wavelengths within the design bandwidth of the microphone—this phase difference tends to increase the pressure difference between the acoustic sensors, lower frequency acoustic waves (which produce a lower phase difference) may experience a higher degree of cancellation in a multi-element dipole microphone than high frequencies.
Speech from the headset wearer also has the characteristic that it arrives at the microphone from a particular fixed direction. This is opposed to ambient noise, which may arrive from any direction. As previously discussed, the dipole microphone's sensitivity to pressure gradients makes it sensitive to acoustic waves arriving along the axis of the microphone; but causes it to produce relatively little output for acoustic waves arriving from the sides. By using a dipole microphone aligned with the headset wearer's mouth, further ambient noise reduction may be achieved due to the dipole microphone having lower sensitivity to ambient sounds arriving from the side.
To function properly as a dipole microphone, the omni-directional sensors must be matched, so that each sensor produces an output signal having the same amplitude and phase as the other sensor when exposed to an acoustic wave producing the same absolute pressure at each sensor. If the dipole pair is not perfectly matched, the differential output will not be zero when both sensors are exposed to equal absolute pressure, and the dipole microphone response will begin to take on the characteristics of an omni-directional microphone. Thus, mismatched sensor pairs will degrade the noise cancelling performance of the dipole microphone by reducing both the microphone's directivity and near field/far field sensitivity ratio.
As a practical matter, a dipole sensor pair is rarely, if ever, perfectly matched due to minor production variations between each sensor. Moreover, measuring and sorting acoustic sensors to select closely matched pairs drives up the cost of the multi-sensor dipole microphone, reducing or eliminating its economic advantage over a single element dipole microphone. In addition, sensors which are closely matched at the time the dipole microphone is produced can nevertheless become mismatched over time from exposure to environmental factors such as temperature variations, moisture, dirt, mechanical shocks from being dropped, as well as from simple aging of the sensors.
Therefore, in order to provide high noise cancelling performance from low cost acoustic sensors, it is necessary to produce matched dipole elements without sorting through numerous sensors. Further, it is desirable that sensor matching be maintained as the microphone ages. Retrieving headsets to verify the noise cancelling performance and calibrate dipole microphones by switching or adjusting components is costly and burdensome, and thus is not a viable solution to the problem of mismatched dipole sensors. Because workers wearing headsets in noisy environments rely on the noise cancelling performance of the headset microphone to maintain communications, new and improved methods and systems for matching microphone elements are needed if dipole microphones using low cost acoustic sensor pairs are to be deployed in the field.